1. Field of the Invention
The present invention relates to a semiconductor laser element having a nitride semiconductor layer and a method for manufacturing the same.
2. Description of Related Art
In these years, an optical disc is requested to have larger and larger memory capacity with higher density. In order to responds to this request, BD (Blue ray Disc) and HD-DVD (High Definition DVD) using a blue color semiconductor laser are standardized, and devices such as decoders conforming the standards have been commercialized. New types of disks for these standards require a high power blue color semiconductor laser with high reliability so as to enable higher density and high speed writing of information on a two-layer disk.
An AlGaAs system or an InGaAlP system semiconductor laser for reproducing or writing information on a conventional CD or DVD includes has a coating film made of a dielectric such as SiO2, Al2O3 or Si3N4 on an end face of resonator for preventing a deterioration or an optical damage to the end face of resonator that is an exit face of a laser beam. However, if an EB (Electron Beam) evaporator or a sputtering device is used for depositing a coating film as it is on the nitride semiconductor laser that is a blue color semiconductor laser, a COD level that is a critical power at which a COD (Catastrophe Optical Damage) may occur is low, so that reliability is very low. Therefore, an improvement in coating technique is necessary. Note that the COD means a phenomenon of melting crystals constituting a semiconductor laser element due to absorption of laser beam by the exit face.
JP-A-2002-335053 discloses a method for manufacturing a semiconductor laser, in which a resonator end face of the semiconductor laser element formed by cleavage is exposed to an argon plasma atmosphere so that a natural oxide film formed on the resonator end face naturally is removed by argon particles in a plasma state. Thus, the adhesion of the coating film formed on the resonator end face to the resonator end face is enhanced so that reliability of the resonator end face is improved. In addition, it is also proposed to remove moisture or the like that adheres to the surface of the semiconductor by heating the same after the cleavage, so as to improve the reliability more.
In the exposure to the argon plasma atmosphere that is used for the method for manufacturing a semiconductor laser as proposed in JP-A-2002-335053, a voltage is not applied to the semiconductor laser bar and a holder thereof, and an argon ion is not attracted to the semiconductor laser bar by a potential difference so as to collide the resonator end face of the semiconductor laser bar. In other words, it is not so-called counter sputtering. In this case, the ion that reaches the resonator end face of the semiconductor laser bar is regarded to have energy of tens of keV. This energy of the ion is sufficient for removing by plasma thereof a moisture, carbon, a natural oxide film or the like adhered to the surface of the semiconductor laser bar, and it has been considered to damage hardly to the resonator end face of the semiconductor laser bar.
The inventor performed an ultimate analysis of the resonator end face that had been exposed to the argon plasma atmosphere for studying effects of the exposure to the argon plasma atmosphere. As a result, the inventor found that carbon and oxygen were not observed when the exposure to the argon plasma atmosphere had been performed though they are observed if the exposure to the argon plasma atmosphere was not performed. The observed carbon and oxygen are considered to be contained in the natural oxide film, moisture or contaminant that adhered during the time period after forming the resonator end face by cleavage until forming the coating film. When a heat treatment had been performed on the resonator end face instead of the exposure to the argon plasma atmosphere, a similar result was obtained.
From this result, it can be said that the formation of the coating film should be performed after removing the carbon and the oxygen from the resonator end face for improving the semiconductor laser element, and that such formation of the coating film can be realized by a heat treatment or exposure to the argon plasma atmosphere.
However, according to an experiment performed by the inventor, the following fact was found. That is, although the exposure to the argon plasma atmosphere is effective for removing the carbon and the oxygen from the resonator end face, the surface of the semiconductor laser element including the resonator end face is affected by the exposure if the semiconductor laser element is a nitride semiconductor laser element.
The detail of this experiment is as follows. Two samples of the nitride semiconductor laser element were manufactured. One of the samples was exposed to the argon plasma atmosphere, while the other sample was not exposed to the argon plasma atmosphere. Each of the samples was subjected to an aging test, and the COD level of each sample was measured before and after the aging test. FIG. 11 shows a variation of the COD levels before the aging and after 200 hours of the aging about two samples. One of the samples has the coating film of Al2O3 formed on the resonator end face after exposure to the argon plasma atmosphere. The other sample has the coating film of Al2O3 formed on the resonator end face that is in the state after the cleavage without exposed to the argon plasma atmosphere. The conditions of the aging include an ambient temperature at 70° C., a power at 60 mW, APC (Automatic Power Control) driving and CW (Continuous Wave) driving. In addition, the COD levels were measured under the condition of 50 ns, duty factor of 50%, at room temperature and a pulse measurement.
As understood from FIG. 11, the COD level before the aging, i.e., an initial COD level is lower for the sample exposed to the argon plasma atmosphere than the other sample. This is considered to be because that the exposure to the argon plasma atmosphere had some influence to the resonator end face.
In addition, the COD levels after the aging of both the samples with the exposure and without the exposure to the argon plasma atmosphere are dropped from the COD levels before the aging. However, the sample that was exposed to the argon plasma atmosphere has a higher COD level than the sample that was not exposed to the argon plasma atmosphere. In other words, the relationship between the two COD levels is inverted after the exposure. Therefore, it can be said that deterioration in the COD level due to aging is reduced by the exposure to the argon plasma atmosphere so that reliability can be improved.
The reason of this can be considered as follows. In the nitride semiconductor laser element that was not exposed to the argon plasma atmosphere, there is an interface state that can cause a non-light emission recombination at the interface between the resonator end face and the coating film due to an impurity such as the natural oxide film or the like in the resonator end face. Therefore, the resonator end face was deteriorated by the heat during the aging. On the contrary, in the nitride semiconductor laser element that was exposed to the argon plasma atmosphere, the interface state that can cause a non-light emission recombination at the interface between the resonator end face and the coating film is reduced compared with the case without the argon plasma atmosphere. Therefore, there is little heat generated during the aging so that the deterioration in the resonator end face is reduced, resulting in little deterioration in the COD level.
In other words, although the exposure to the argon plasma atmosphere has an advantage that deterioration in the COD level can be reduced, it also has a disadvantage that the initial COD level is decreased. The decrease of the initial COD level may be a serious problem when a high power of the nitride semiconductor laser element is required. For example, in the case of the semiconductor laser element having characteristics shown in FIG. 11, it is difficult to realize a high power above the initial COD level at 200 mW for the one that was exposed to the argon plasma atmosphere.
Although the exposure to the argon plasma atmosphere may cause some damage to the resonator end face as described above, detail or a specific content of the damage has not been studied up to now. Therefore, the inventor studied about it and found that the exposure to the argon plasma atmosphere causes reduction of quantity of nitrogen in the resonator end face made up of a nitride semiconductor. The detail of this will be described below.
Concerning the nitride semiconductor laser element, a ratio of the number of atoms between gallium and nitrogen was measured by an AES (Auger Electron Spectroscopy) method while sputter etching was performed on the resonator end face from the surface toward the inside. The measurement was performed in the area where GaN is exposed on the surface. FIG. 12 shows the measurement result of the sample without the exposure to the argon plasma atmosphere and the sample with the exposure to the argon plasma atmosphere. The vertical axis corresponds to the ratio of the number of nitrogen atoms to that of gallium atoms, while the horizontal axis corresponds to a time period of the sputter etching. Hereinafter, in this specification, every description about a ratio of nitrogen to gallium means the ratio of the number of atoms. Here, one minutes of the sputter etching corresponds to a depth of approximately 3 nm. As understood from FIG. 12, the exposure to the argon plasma atmosphere caused a reduction of the ratio of nitrogen to gallium at the top surface of the nitride semiconductor laser element (at zero in the horizontal axis in FIG. 12).
Further, concerning another nitride semiconductor laser element, the ratio of the number of atoms between gallium and nitrogen was measured by the AES method while sputter etching was performed on the resonator end face from the surface toward the inside. FIG. 13 shows the measurement result of the sample without the exposure to the argon plasma atmosphere and the sample with the exposure to the argon plasma atmosphere. The horizontal axis corresponds to a depth from the surface of the resonator end face, while the vertical axis corresponds to the ratio of nitrogen to gallium. It is understood from FIG. 13 too that the exposure to the argon plasma atmosphere caused a reduction of the ratio of nitrogen to gallium at the top surface of the nitride semiconductor laser element.
As described above, the ratio of nitrogen to gallium is reduced on the surface of the nitride semiconductor laser element that was exposed to the argon plasma atmosphere, and the reason of this is considered to be removal of nitrogen having a high vapor pressure due to an attack of the excited argon ions. In addition, this reason may include that the exposure to the argon plasma atmosphere causes removal and reduction of the nitrogen from the surface of the resonator end face so that the resonator end face becomes the state where gallium exists more than nitrogen. Since stoichiometry in which a ratio of nitrogen to gallium is balanced as 1:1 is imbalanced substantially, a non-light emission center increases resulting in an increase of heat value that causes a rapid deterioration in the end face. For this reason, the state of removing nitrogen depends on a time period of the exposure to the argon plasma atmosphere, a power of a micro wave, a treatment temperature, and the like.
As shown in FIG. 11, the sample that was exposed to the argon plasma atmosphere has the lower initial COD level than the sample that was not exposed to the argon plasma atmosphere. This is considered to be resulted from that the removal of the nitrogen from the resonator end face brings about an increase of the non-light emission center as well as a probability of the non-light emission recombination so that the heat value increases.
Furthermore, in the method for manufacturing a semiconductor laser as proposed in JP-A-2002-335053, the semiconductor laser bar is heated for evaporating and removing the moisture adhered to the end surface before forming the coating film. This process with heating the semiconductor laser bar to a temperature higher than the room temperature is usually performed for evaporating moisture or improving quality of the coating film. However, in the case of the nitride semiconductor laser, it was found that this heating process causes removal of nitrogen from the resonator end face resulting in lowering of the COD level.